1. Field of the Invention
The present invention relates generally to telecommunications systems, for example wireless digital cellular systems, employing complex or quadrature modulated information signals containing data organized into successive time slots, each slot containing a series of pilot bits and a series of data bits, and more specifically to an apparatus and method for channel estimation and de-rotation of such an information signal received via a channel.
2. Description of the Related Art
In planned third generation digital wireless cellular systems known as Uniform Mobile Telephone System (UMTS), Wideband Code Division Multiple Access (W-CDMA), and Third Generation (e.g. 3G Partnership Project) spread spectrum information signals are used which contain data grouped into slots, each slot consisting of a predetermined series of Npilot pilot bits in a first portion of the slot and a series of Ndata data bits in the second portion of the slot. It is known to use various types of filtering schemes, ranging from simple to complex, to achieve channel estimation and de-rotation of a despread received spread spectrum information signal by synchronizing with the pilot bits. The nature of the estimation error achieved with prior art filtering schemes varies, but generally the error propagates from slot, sometimes increasing over time.
While decision feedback loops are known for other purposes, the prior art has not considered the possibility of using a decision feedback loop for channel estimation and de-rotation of a spread spectrum signal received via a channel.
It is an object of the present invention to provide a decision feedback loop apparatus and method for a receiver for channel estimation and de-rotation of a received signal. It is a further object that the decision feedback loop is implemented in a manner to mitigate propagation of the estimation error from slot to slot.
This and other objects of the present invention are satisfied by a decision feedback loop which uses the known sequence of pilot bits to initialize and train the feedback loop during each slot. This continued sloe by slot re-initialization and re-training prevents the estimation error in one slot from propagating to the next.
In accordance with the invention, a decision feedback loop apparatus for a receiver for channel estimation and de-rotation of complex input symbols derived from a sampled information signal received via a channel, which information signal contains data organized into successive time slots, each slot containing a predetermined series of pilot bits during a first portion of the slot and a series of data bits during a second portion of the slot, comprises a first multiplier for multiplying the complex input symbols with estimated conjugate channel coefficients, which are derived from a feedback signal, to form complex soft symbols to be used for channel decoding, a hard decision device for forming complex hard symbols from the complex soft symbols, a pilot generator for generating a series of complex pilot symbols corresponding to the predetermined series of pilot bits in the information signal, and a second multiplier for multiplying a first signal at a first input, which is derived from the complex input signal, with a second signal at a second input to form a feedback signal at an output. The invention is characterized in that the second signal is derived from the complex pilot symbols during the first portion of the slot and from the complex hard symbols during the second portion of the slot.
Similarly, in accordance with the invention, a decision feedback loop method for a receiver for channel estimation and de-rotation of complex input symbols derived from a sampled information signal received via a channel, which information signal contains data organized into successive time slots, each slot containing a predetermined series of pilot bits during a first portion of the slot and a series of data bits during a second portion of the slot, comprises multiplying the complex input symbols with estimated conjugate channel coefficients, which are derived from a feedback signal, to form complex soft symbols to be used for channel decoding, forming complex hard symbols from the complex soft symbols, generating a series of complex pilot symbols corresponding to the predetermined series of pilot bits in the information signal, and multiplying a first signal, which is derived from the complex input signal, with a second signal to form a feedback signal. The inventive method is characterized by the act of deriving the second signal from the complex pilot symbols during the first portion of the slot and from the complex hard symbols during the second portion of the slot.
Another aspect of the invention is that the estimated channel coefficients are derived by applying a filter to the feedback signal.
Other objects, features and advantages of the present invention will become apparent upon perusal of the following detailed description when taken in conjunction with the appended drawing, wherein: